1. Field of the Invention
The invention relates generally to strain gauges and more particularly to a strain gauge deposited upon a substrate by thin film techniques exhibiting very high sensitivity to any change in length or electrical conductivity.
2. Description of the Prior Art
Conventional strain gauges employ a plurality of metal-wire or metal-foil resistance elements or are constructed by thin-film techniques. They are used as sensors for measuring the parameters force, distance, weight, acceleration or pressure. The most commonly used train gauges are piezoelectric semiconductor crystals as described in German Patent DE-AS No. 23 49 463. They can be used to measure the piezoelectric effect directly and the conductivity change caused by the piezoelectric effect. The strain is, in most cases, mounted indirectly by cementing or screwing the crystal to a substrate to be tested. Exceptions are those strain gauges which are made by thin-film techniques but like all piezocrystals, must be oriented according to their structure. As a rule, the indirect mounting causes a hysteresis. In addition to the hysteresis of the piezoelectric effect, a measurement inaccuracy results between elongation and compression. The crystalline structure of the strain gauge which is easily broken does not permit any sharp bending of the piezoelectric crystal. This problem limits the range of application of the piezoelectric semiconductor crystals.
In measuring the piezoelectric effect, fast aging is particularly disadvantageous because it necessitates continuous recalibration. Very reliable strain gauges are elements which are formed by a metal strip and in which strain causes a resistance change as a result of a change in the cross-section and the length of the strain gauge. This change is very small so that the strain measurement obtained is not very accurate.